Migration refactorings after renaming from `.js` to `.ts` files

[DanielRosenwasser]

At least week's design meeting, @sheetalkamat had the idea that we should consider applying refactorings for when you rename a .js file to a .ts file.

I think this is definitely worth experimenting with. From the refactoring pipeline, I could imagine things like

• moving from CommonJS to ES Modules
• lifting JSDoc annotations
• moving to classes
• inferring from usages

Then there's project configuration. I could see

• Generating a tsconfig.json
• Installing @types dependencies for packages that need them.

@mjbvz @amcasey